The present invention relates to battery state-of-charge (SOC) estimation, and more particularly to methods and apparatus for calibrating coulomb counting based SOC estimation.
Batteries are used in numerous applications, such as portable devices, smartphones, laptops, tablets and so on. It is important for the user to know an amount of available energy remaining in a battery and when the battery needs to be charged, so as to avoid the battery running out of energy.
An amount of charges remained in the battery is usually expressed by state of charge (SOC), which is in form of a percentage of a maximum battery capacity. Usually, 100% SOC state means a battery cell is fully charged, while 0% SOC state means the battery cell is fully discharged.
There are several approaches to estimate the SOC of the battery cell. One of the approaches is open circuit voltage (OCV)-based SOC estimation that is mainly based on an open circuit voltage (OCV) of the battery cell. This approach measures potential difference between terminals of the battery cell to obtain the OCV and translate the measured OCV into the SOC according to a known OCV-SOC curve, which defines relationship between OCV and SOC values. However, due to an internal impedance of the battery cell, it is difficult to accurately measure the OCV especially when a current flowing through the battery cell is high. Another approach is to integrate a current flowing into or out of the battery cell to count a total charge amount that have flown through the battery cell, thereby estimating a relative SOC change within a given period of time, which is as known as coulomb counting. However, as the coulomb counting only estimates the relative SOC change, it is necessary to estimate an initial value, which is the SOC value at the time the current starts to be integrated. Accordingly, by applying the relative SOC change to the initial value, the actual SOC can be therefore determined.
Even when the above-mentioned coulomb counting based SOC estimation is considerably accurate when compared to OCV-based SOC estimation, there are still errors in the coulomb counting based SOC estimation. This is because the SOC of a battery cell is dependent on its inherent chemical characteristics and characteristics of the electrical system that draws power from the battery cell. Lot of reasons, such as battery aging or battery temperature may lead to inaccurate estimation of the initial value. Furthermore, a change of a loading on the battery cell also causes the error in estimating the relative SOC change because the change of the loading on the battery cell affects a maximum battery capacity, which further affects the computation of the relative SOC change. In view of this, the coulomb counting based SOC estimation requires to be calibrated on a regular basis to guarantee an accurate SOC estimation.